1. Field of the Invention
This invention relates to an apparatus and method for providing redundant protection to a fault detection/interruption circuit, thereby ensuring safe operation even in the case of a failure of the primary fault detection/interruption means. Upon the occurrence of a failure in the primary circuit interruption means, a secondary circuit breaker, or in some embodiments, a redundant primary circuit breaker release mechanism, serves to remove power from a protected outlet or output conductors.
2. Background of the Invention
A common source of electrical injuries occurs when an accidental electrical leakage from one electrified object to a second object having a substantially different voltage potential occurs, with the electrical leakage passing through a human. When one of the two electrified objects is at the same potential as the earth (or so-called ground), this is called a ground fault. A circuit to protect against injury due to ground faults is called a ground fault circuit interrupter or GFCI. These devices are built into the electrical outlets of many homes and businesses and, in particular, are required by code in the U.S. for bathrooms and outdoor outlets in new construction.
GFCI""s are not immune from failure. In the U.S., the 2001 GFCI Field Test Survey Report by the National Electrical Manufacturers Association found that an estimated 14% of circuit breaker GFCI""s and 8% of receptacle GFCI""s in the field are not operational. As the installed base of GFCI circuits ages, this percentage will increase. The finding of such a large percentage of non operational GFCI""s has led to a great deal of concern about unprotected power. Clearly, any circuit improvements that can enhance the robustness of GFCI devices will serve to reduce the potential for electrical injury.
The present invention combines a secondary circuit breaker with a standard fault interrupter. This standard fault interrupter can be a ground fault circuit interrupt (GFCI) or one of the derivative fault interrupters including, but not limited to, arc fault circuit interrupt (AFCI), immersion detection circuit interrupt (IDCI), leakage current detect and interrupt (LCDI) or appliance leakage circuit interrupt (ALCI). The secondary circuit breaker is triggered some interval after certain events such as a sensed fault or a manual test. This secondary circuit breaker receives its power from a point that should have no power if the fault detection and interrupt mechanism is correctly functioning. Consequently, if the fault detection/interruption circuit works satisfactorily, then the secondary circuit breaker is never fired. In its preferred embodiment, the secondary breaker would be a one-shot circuit breaker, serving to permanently remove power from the output and forcing the user to replace the malfunctioning unit. One shot circuit breakers are designed as normally closed switches, which, when activated, open permanently.
There are a variety of circuit interruption means that comprise the class of one-shot circuit breakers. The most common example is a thermal fuse, whereby two electrical conductors are in electrical contact through a low melting point linkage that opens when the current flow exceeds a certain threshold. U.S. Pat. No. 3,629,766 (Gould) describes a circuit breaker wherein a fusible wire link holds spring biased conductors in a closed position. When a predetermined electrical current is passed through the fusible link it causes it to break, effecting the snap action release of the spring arms and breaking the electrical connection. Other examples of circuit interruption means include the one-shot breaker described in U.S. Pat. No. 5,394,289 (Yao and Keung) wherein wire fuses connect two sets of two conductors. A current overload is used to break one fuse, whereupon, a cutting element is released to cut through the other fuse. U.S. Pat. No. 4,829,390 (Simon) describes a switch that is held in a normally closed position by a flash bulb. A sensor detects a dangerous condition and actuates the flash bulb, causing it to disintegrate and allowing the switch to open. Bimetallic thermal and thermal magnetic circuit breakers are well known in the art and are the basis for many resettable circuit breakers, although they can be used for one-shot operation. These work by employing a blade made of two metals having different thermal coefficients of expansion. When the blade is heated, it deforms, breaking a circuit. The magnetic breakers use heating to reduce the magnetic attraction of a magnet, thereby causing a spring loaded contact to release and open a circuit. Other designs for circuit breakers include piezoelectric actuators as in U.S. Pat. No. 4,473,859 (Stone et al) and shape memory alloy actuators as in U.S. Pat. No. 3,403,238 (Buehler and Goldstein).
U.S. Pat. No. 6,262,871 B1 (Nemir et al) discloses an electronic test circuit for the self-testing of fault detection devices. This self-test circuit enhances the safety of such devices by periodically and automatically testing the function of the fault detection portion of the device without the need for manual intervention. By using a secondary circuit breaker, power may be safely and automatically removed from a malfunctioning fault detection device. One problem with this device is that the self-test circuit has a complexity that is of a higher order than that of the original fault detection/interruption electronics, thereby adding to overall system complexity and cost.
U.S. Pat. Nos. 6,282,070 B1 (Ziegler et al), 6,288,882 B1 (DiSalvo et al), and 6,381,112 B1 (DiSalvo) all disclose a fault detection/interruption device having a so-called xe2x80x9creset lockoutxe2x80x9d. With a reset lockout, the electrical connections between input and output conductors are said to be prevented from resetting if the circuit interruption mechanism is non-operational or if an open neutral condition exists. However, these inventions have no means for self-test during normal operation. For example, if the fault detection component fails at some time during use, this failure will go undetected until such time as a manual test is implemented. Since there is no way to ensure that a periodic manual test is implemented, this approach can result in unprotected power being furnished at the outlet or over the branch wiring that connects the reset lockout equipped GFCI to an electrical outlet. Furthermore, some failure modes, such as welded circuit breaker contacts, will be undetected and uncorrected by these inventions.
3. Objects and Advantages
The present invention is designed to be easily added to, or integrated within, an existing technology GFCI circuit and to operate independently of that circuit. The present invention serves as an auxiliary tester that causes the overall device to fail safe in the event of a failure in the GFCI. One major advantage to the proposed invention is that it is inexpensive and can be added to an existing ground fault circuit interrupter, thereby taking advantage of existing technology while improving robustness. A second advantage is that it can automatically detect a malfunctioning electrical current interruption means and can cure that event by firing a secondary circuit breaker, thereby removing power from the system. Alternatively, rather than employing a completely independent secondary circuit breaker, some embodiments may utilize a combined release mechanism on a single, primary circuit breaker, with fail safe protection provided by a redundant, independently controlled, auxiliary circuit breaker release.
The present invention is a fail safe fault interrupter that consists of a conventional GFCI with either a second circuit breaker and a second circuit breaker trigger, or an auxiliary circuit breaker release mechanism. When either (a) a fault is sensed; or (b) a manual test is engaged; the second circuit breaker is triggered with a time delayed signal that takes its power from the load side of the fault interrupter. Accordingly, if the power to a fault is satisfactorily interrupted within a designated time after the application of either a manual test or a sensed fault, then there will be no power available to trigger the second circuit breaker (alternatively, the auxiliary circuit breaker release mechanism) and this second circuit breaker will remain in a closed position. Alternatively, if the power is not removed within the designated time interval, the secondary breaker will be opened, thereby removing power from the system.